Photovoltaic cell and photovoltaic cell manufacturing method

ABSTRACT

A photovoltaic cell manufacturing method includes depositing a first buffer layer for performing lattice relaxation on a first silicon substrate; depositing a first photoelectric conversion cell on the first buffer layer, the first photoelectric conversion cell being formed with a compound semiconductor including a pn junction, and the first photoelectric conversion cell having a lattice constant that is higher than that of silicon; connecting a support substrate to the first photoelectric conversion cell to form a first layered body; and removing the first buffer layer and the first silicon substrate from the first layered body.

TECHNICAL FIELD

The present invention relates to a photovoltaic cell manufacturingmethod.

BACKGROUND ART

Compound semiconductors have different levels of band gap energy andlattice constants according to the material composition. Therefore, amulti-junction photovoltaic cell is produced, by which the wavelengthrange of sunlight is divided among a plurality of photovoltaic cells sothat the energy conversion efficiency is increased.

Presently, a typical example of a multi-junction photovoltaic cell is atriple-junction photovoltaic cell (1.9 eV/1.4 eV/0.67 eV) including Gecell/Ga(In)As cell/GaInP cell using a lattice matching material,provided on a germanium (Ge) substrate having substantially the samelattice constant as that of gallium arsenide (GaAs).

The efficiency of a photovoltaic cell made of a compound semiconductoris approximately two times as high as that of a silicon (Si)photovoltaic cell. However, a photovoltaic cell made of a compoundsemiconductor has a high-cost substrate or a small-sized substrate, andis thus significantly more expensive than a silicon photovoltaic cell.Accordingly, a photovoltaic cell made of a compound semiconductor isused for special purposes, mainly for use in space.

Furthermore, recently, a concentrated photovoltaic cell is formed bycombining an inexpensive condensing lens made of plastic and a smallcell of a photovoltaic cell made of a compound semiconductor.Accordingly, the usage amount of an expensive compound semiconductor isreduced compared to a typical flat plate photovoltaic cell formedwithout using a condensing lens. Such a concentrated photovoltaic cellcan be manufactured at a lower cost and is used practically as aphotovoltaic cell for general purposes other than special purposes asdescribed above.

However, the power generation cost of a photovoltaic cell still remainshigh, and therefore it is imperative to further reduce the cost. Thus,studies are being conducted to increase the energy conversion efficiencyand to reduce the manufacturing cost.

As an example of reducing cost, studies are being conducted to produce aphotovoltaic cell with a compound semiconductor on a Si substrate thatcosts less by approximately one digit and whose area can be made large(see, for example, Non-patent Document 1). However, a Si substrate and aphotovoltaic cell made of a compound semiconductor have differentlattice constants, and a dislocation can occur due to latticerelaxation. Thus, a buffer layer is provided between the Si substrateand the photovoltaic cell layer for relaxing the lattice constant(performing lattice relaxation), in order to cause the difference in thelattice constant to relax as much as possible in the buffer layer, andtherefore reduce the dislocation in the compound semiconductor.

Furthermore, there is proposed a method of forming a photovoltaic celllayer on each of the Si substrate and the GaAs substrate, pasting thesetogether by a direct bonding method and removing the GaAs substrate, andforming a double-junction photovoltaic cell on the Si substrate (see,for example, Non-patent Documents 2 and 3).

Furthermore, there is proposed a method of manufacturing a photovoltaiccell by a smart cut method which involves implanting H+ ions, etc.,inside a semiconductor substrate and peeling off a thin-layer from thesubstrate starting from the part where ions have been implanted. Afterimplanting the ions, a Si substrate is bonded together with a Gesubstrate, a GaAs substrate, or an InP substrate via SiO₂. Then, by aheating process, the Ge substrate, the GaAs substrate, or the InPsubstrate is peeled off, and a photovoltaic cell made of a compoundsemiconductor is formed on a template substrate constituted by a Gelayer, a GaAs layer, or an InP-layer provided on the Si substrate (see,for example, Non-patent Documents 4, 5, and 6).

However, by the above methods of manufacturing a photovoltaic cell madeof a compound semiconductor, an expensive GaAs substrate or InPsubstrate is used, and therefore the photovoltaic cell made of acompound semiconductor cannot be manufactured at a low cost.

As described above, by conventional manufacturing methods, thephotovoltaic cell made of a compound semiconductor cannot bemanufactured at a low cost.

-   Non-patent Document 1: Yamaguchi et al, Proceedings of the 28th IEEE    Photovoltaic Specialists Conference (2002), pp. 860-863-   Non-patent Document 2: The Japan Society of Applied Physics Autumn    proceedings, 2010, 15p-NC-4-   Non-patent Document 3: The Japan Society of Applied Physics Spring    proceedings, 2012, 17p-DP3-6-   Non-patent Document 4: Appl. Phys. Lett. 92, 103503, (2008)-   Non-patent Document 5: Proceedings of the IEEE 4th World Conference    on Photovoltaic Energy Conversion (2006), pp. 776-779.-   Non-patent Document 6: Appl. Phys. Lett. 91, 012108, (2007)-   Patent Document 1: Japanese Laid-Open Patent Publication No.    S61-219182-   Patent Document 2: Japanese Laid-Open Patent Publication No.    2006-216896

DISCLOSURE OF INVENTION

The present invention has been made in view of the above-describedproblems, and it is an object of at least one embodiment of the presentinvention to provide a photovoltaic cell manufacturing method formanufacturing a photovoltaic cell made of a compound semiconductor, at alow cost.

An aspect of the present invention provides a photovoltaic cellmanufacturing method which includes depositing a first buffer layer forperforming lattice relaxation on a first silicon substrate; depositing afirst photoelectric conversion cell on the first buffer layer, the firstphotoelectric conversion cell being formed with a compound semiconductorincluding a pn junction, and the first photoelectric conversion cellhaving a lattice constant that is higher than that of silicon;connecting a support substrate to the first photoelectric conversioncell to form a first layered body; and removing the first buffer layerand the first silicon substrate from the first layered body.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B illustrate a photovoltaic cell manufacturing methodaccording to a first embodiment;

FIGS. 2A and 2B illustrate the photovoltaic cell manufacturing methodaccording to the first embodiment;

FIG. 3 illustrates the photovoltaic cell manufacturing method accordingto the first embodiment;

FIG. 4 illustrates a layered body according to a modification of thefirst embodiment, in which a Si substrate and a buffer layer are removedby a lift off method;

FIGS. 5A and 5B illustrate a smart cut method according to amodification of the first embodiment;

FIGS. 6A and 6B illustrate a photovoltaic cell manufacturing methodaccording to a second embodiment;

FIGS. 7A and 7B illustrate the photovoltaic cell manufacturing methodaccording to the second embodiment;

FIG. 8 is a cross-sectional view of a photovoltaic cell according to amodification of the second embodiment;

FIG. 9 illustrates a photovoltaic cell manufacturing method according toa third embodiment;

FIG. 10 illustrates the photovoltaic cell manufacturing method accordingto the third embodiment;

FIGS. 11A and 11B illustrate the photovoltaic cell manufacturing methodaccording to the third embodiment;

FIGS. 12A and 12B illustrate the photovoltaic cell manufacturing methodaccording to the third embodiment;

FIG. 13 illustrates the photovoltaic cell manufacturing method accordingto the third embodiment;

FIGS. 14A and 14B illustrate a photovoltaic cell manufacturing methodaccording to a fourth embodiment;

FIGS. 15A and 15B illustrate the photovoltaic cell manufacturing methodaccording to the fourth embodiment; and

FIG. 16 illustrates the photovoltaic cell manufacturing method accordingto the fourth embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described below with referenceto the accompanying drawings.

First Embodiment

FIGS. 1A through 3 illustrate a photovoltaic cell manufacturing methodaccording to a first embodiment.

First, as illustrated in FIG. 1A, a buffer layer 11, a contact layer 12,a GaInP cell 20, a tunnel junction layer 13, a GaAs cell 30, and acontact layer 14, are sequentially formed on a Si substrate 10. Alayered body 100 illustrated in FIG. 1A is made assuming that sunlightenters from the bottom side as viewed in FIG. 1A.

The Si substrate 10 may be, for example, a substrate made of a non-dopedsilicon single crystal. As the Si substrate 10, for example, a siliconwafer having a size of 8 inches or 12 inches may be used. Note that theSi substrate 10 is not limited to the above, and any kind of substratemay be used.

The buffer layer 11 is formed by depositing, on one side of the Sisubstrate 10, two layers, i.e., a silicon germanium (SiGe) layer and agermanium (Ge) layer. The SiGe layer and the Ge layer may be formed by,for example, a CVD (Chemical Vapor Deposition) method.

The SiGe layer includes a defect that is caused by the difference in thelattice constant of the Si crystal and the Ge crystal, and therefore theSiGe layer becomes a layer in which the difference in the latticeconstant is relaxed (lattice relaxation is performed). The SiGe layer isdeposited on the Si substrate 10 in order to relax the differencebetween the lattice constant of the compound semiconductor layer to beformed later and the lattice constant of the Si substrate 10.Furthermore, the Ge layer is deposited on the SiGe layer, because the Gelayer has a lattice constant that is close to that of the compoundsemiconductor layer to be formed later.

As described above, the difference in the lattice constant is relaxed inthe SiGe layer, and therefore the Ge layer formed on the SiGe layer isgrown with hardly any distortion. Due to the above reasons, on the backside of the Si substrate 10, a SiGe layer is formed, and on the backside of the buffer layer 11, a Ge layer is formed.

Note that the buffer layer 11 may not be formed to have a two layerstructure of a SiGe layer and a Ge layer, but may be formed to have astructure in which the ratio of silicon and germanium is continuouslychanged. For example, first, a SiGe layer having a high silicon ratiomay be formed on one side of the Si substrate 10, and the ratio of Gemay be gradually increased so that a Ge layer is attained on the backside, to form the buffer layer 11.

The contact layer 12 is mainly a layer to be deposited on the bufferlayer 11 for ohmic-connecting with a metal layer 17 (see FIG. 3) to beformed later, and for example, a gallium arsenide (GaAs) layer is usedas the contact layer 12. For example, the GaAs layer used as the contactlayer 12 may be formed on the buffer layer 11 by a MOCVD (Metal OrganicChemical Vapor Deposition) method.

The GaAs layer used as the contact layer 12 has a lattice constant thatis significantly close to that of the Ge layer positioned at the topmostlayer of the buffer layer 11. Therefore, it is possible to cause crystalgrowth of a GaAs layer used as the contact layer 12, on the buffer layer11. Note that it is possible to cause crystal growth of a GaInAs layerhaving a composition that lattice-matches Ge, as the contact layer 12.

The GaInP cell 20 is a photoelectric conversion cell made of a compoundsemiconductor, including gallium (Ga), indium (In), and phosphorus (P)as raw materials. The GaInP cell 20 includes an n-layer 21 and a p-layer22. For example, the GaInP cell 20 is formed by a MOCVD method, bysequentially depositing the n-layer 21 and the p-layer 22 on the contactlayer 12.

As the dopant of the n-layer 21, for example, silicon (Si) or selenium(Se) may be used. Furthermore, as the dopant of the p-layer 22, forexample, zinc (Zn) or magnesium (Mg) may be used.

In the first embodiment, the composition of Ga, In, and P is adjusted sothat the band gap of the GaInP cell 20 becomes 1.9 eV. Furthermore, theGaInP cell 20 has a lattice constant that is significantly close to thelattice constant (approximately 5.65 Å) of the GaAs layer used as thecontact layer 12. The composition of Ga, In, and P is adjusted so thatthe GaInP cell 20 can crystal-grow on the contact layer 12.

Note that between the n-layer 21 and the contact layer 12 (incident sideto the GaInP cell 20), a window layer having a wider gap than that ofthe GaInP cell 20 may be formed. Furthermore, above the p-layer 22(between the p-layer 22 and the tunnel junction layer 13), a BSF (BackSurface Field) layer having a wider gap than that of the GaInP cell 20may be formed.

The tunnel junction layer 13 is provided between the GaInP cell 20 andthe GaAs cell 30, and includes an n-layer and p-layer that have beendoped at a higher density than that of the n-layer 21 and the p-layer 22of the GaInP cell 20 and an n-layer 31 and a p-layer 32 of the GaAs cell30. The tunnel junction layer 13 is a junction layer, which is providedso that a current flows between the p-layer 22 of the GaInP cell 20 andthe n-layer 31 of the GaAs cell 30 (by tunnel junction).

For example, the tunnel junction layer 13 is formed by a MOCVD method,by sequentially depositing an AlGaAs layer (p-layer) and a GaInP-layer(n-layer) in the stated order on the surface of the GaInP cell 20. Thep-layer and the n-layer of the tunnel junction layer 13 are preferablyformed with a material having a wider band gap than that of the GaInPcell 20. This is to prevent the light that has been transmitted throughthe GaInP cell 20 from being absorbed at the tunnel junction layer 13.

Note that the tunnel junction layer 13 has a lattice constant that issignificantly close to the lattice constant (approximately 5.65 Å) ofthe GaAs layer. The composition of Ga, In, and P is adjusted so that thetunnel junction layer 13 can crystal-grow on the GaInP cell 20.

The GaAs cell 30 is a photoelectric conversion cell made of a compoundsemiconductor, including gallium (Ga) and arsenic (As) as raw materials.The GaAs cell 30 includes the n-layer 31 and the p-layer 32. Forexample, the GaAs cell 30 is formed by a MOCVD method, by sequentiallydepositing the n-layer 31 and the p-layer 32 on the tunnel junctionlayer 13.

As the dopant of the n-layer 31, for example, silicon (Si) or selenium(Se) may be used. Furthermore, as the dopant of the p-layer 32, forexample, zinc (Zn) or magnesium (Mg) may be used.

Note that the GaAs cell 30 has a lattice constant of approximately 5.65Å, and therefore the GaAs cell 30 can crystal-grow on the tunneljunction layer 13.

Note that between the n-layer 31 and the tunnel junction layer 13(incident side to the GaAs cell 30), a window layer having a wider gapthan that of the GaAs cell 30 may be formed. Furthermore, above thep-layer 32 (between the p-layer 32 and the contact layer 14), a BSF(Back Surface Field) layer having a wider gap than that of the GaAs cell30 may be formed.

The contact layer 14 is mainly a layer to be deposited on the GaAs cell30 for ohmic-connecting with an electrode to be formed later (metallayer 15A), and for example, a gallium arsenide (GaAs) layer is used asthe contact layer 14. For example, the GaAs layer used as the contactlayer 14 may be formed on the GaAs cell 30 by a MOCVD method.

Note that the contact layer 14 is constituted by a GaAs layer, andtherefore the lattice constant is approximately 5.65 Å, and cancrystal-grow on the GaAs cell 30.

As described above, the layered body 100 illustrated in FIG. 1A isformed by sequentially depositing, on the Si substrate 10 and the bufferlayer 11, the contact layer 12 that constitutes the light incident side,the GaInP cell 20, the tunnel junction layer 13, the GaAs cell 30, andthe contact layer 14, in the stated order.

Accordingly, the GaInP cell 20 (1.9 eV) having a wide band gap isdisposed closer to the light incident side, than the GaAs cell 30 (1.4eV). This is to absorb the short wavelength light at the GaInP cell 20on the light incident side, and to absorb the light having a relativelylong wavelength on that has been transmitted through the GaInP cell 20,at the GaAs cell 30.

Next, as illustrated in FIG. 1B, a support substrate 80 is prepared, andthe metal layer 15A is formed on the contact layer 14, and a metal layer15B is formed on the support substrate 80. The metal layers 15A, 15B arethin films made of a metal such as gold (Au) or silver (Ag), and may beformed by a vapor deposition method or a sputtering method. Furthermore,the support substrate 80 may be, for example, a film made of plastic.

Note that the object formed by forming the metal layer 15A on thecontact layer 14 of the layered body 100 illustrated in FIG. 1A isreferred to as a layered body 100A.

Next, as illustrated in FIG. 2A, the layered body 100A is turned upsidedown, and a bonding layer 16 is used to bond the metal layer 15A of thelayered body 100A with the metal layer 15B formed on the surface of thesupport substrate 80.

The bonding layer 16 is formed by, for example, applying, on the surfaceof the metal layer 15A or the metal layer 15B, a joining material suchas a conductive epoxy agent formed, for example, by including silver(Ag) nanoparticles in epoxy resin. The bonding layer 16 is formed by,for example, applying the joining material on the surface of the metallayer 15A or the metal layer 15B by screen printing.

A layered body 100B as illustrated in FIG. 2A is formed by joiningtogether the metal layer 15A and the metal layer 15B by using thebonding layer 16 as described above. In this case, the bonding layer 16is heated so that the metal layer 15A and the metal layer 15B are fusedtogether by the bonding layer 16. The layered body 100B includes thelayered body 100A, the bonding layer 16, the metal layer 15B, and thesupport substrate 80.

Note that the metal layer 15A, the bonding layer 16, and the metal layer15B are used as the bottom electrode (backside electrode) of thephotovoltaic cell.

Next, the Si substrate 10 is removed from the layered body 100B byetching the Si substrate 10 and the buffer layer 11 of the layered body100B illustrated in FIG. 2A. Accordingly, a layered body 100C asillustrated in FIG. 2B is formed. The layered body 100C is formed byremoving the Si substrate 10 and the buffer layer 11 from the layeredbody 100B illustrated in FIG. 2A.

The etching of the Si substrate 10 and the buffer layer 11 is performed,for example, by a wet etching process using an etching solutionincluding hydrofluoric acid, nitric acid, or acetic acid. The etchingsolution may be a mixed solution including hydrofluoric acid, nitricacid, and acetic acid. The composition of the etching solution may beappropriately determined according to the composition ratio of Si and Geincluded in the buffer layer 11.

Note that the etching solution does not dissolve the contact layer 12(GaAs layer), the GaInP cell 20, the tunnel junction layer 13(AlGaAs/GaInP), the GaAs cell 30, the contact layer 14, the metal layer15A, the bonding layer 16, the metal layer 15B, or the support substrate80. Thus, the wet etching can be performed on the Si substrate 10 andthe buffer layer 11 by impregnating the layered body 100B in the etchingsolution.

Note that the etching is to be performed by, for example, protecting,with a protective layer, the side surfaces of the Si substrate 10, thebuffer layer 11, the contact layer 12, the GaInP cell 20, the tunneljunction layer 13, the GaAs cell 30, the contact layer 14, the metallayer 15A, the bonding layer 16, the metal layer 15B, and the supportsubstrate 80, and the surface of the support substrate 80 (bottomsurface in FIG. 2A). A protective layer will not be used when there isno need for protection in the above etching process.

Furthermore, in this case, a description is given of an embodiment inwhich both the Si substrate 10 and the buffer layer 11 are removed byetching. However, by removing the buffer layer 11 with the use of asolution for selectively etching only the buffer layer 11, the Sisubstrate 10 may be removed from the layered body 100B. As such anetching solution, for example, a nitrohydrofluoric acid solution (amixed solution of hydrofluoric acid and nitric acid) may be used. Notethat water may be added to the nitrohydrofluoric acid solution to dilutethe solution.

Lastly, a metal layer 17 is formed on the top surface of the contactlayer 12 of the layered body 100C. The metal layer 17 is formed on partof the top surface of the contact layer 12. This is because the metallayer 17 becomes the top electrode of the photovoltaic cell.

The metal layer 17 may be formed by, for example, a lift off method. Themetal layer 17 is formed by forming a resist on the top surface of thecontact layer 12 illustrated in FIG. 2B, in areas other than the areawhere the metal layer 17 illustrated in FIG. 3 is to be formed,vapor-depositing metal such as Au or Ag from above the resist, and thenremoving the resist.

Note that a contact layer 12C illustrated in FIG. 3 is formed by formingthe metal layer 17 (see FIG. 3) on the top surface of the contact layer12 illustrated in FIG. 2B, and then using this metal layer 17 as a maskin removing parts of the contact layer 12 (see FIG. 2B) other than thepart positioned immediately below the metal layer 17.

The contact layer 12C may be made by, for example, using a mixed liquidincluding sulfuric acid (H₂SO₄), hydrogen peroxide (H₂O₂), and water(H₂O), as the wet etching solution. The mixed liquid including sulfuricacid (H₂SO₄), hydrogen peroxide (H₂O₂), and water (H₂O) does notdissolve the GaInP in the GaInP cell 20, and therefore the wet etchingprocess can be stopped at the GaInP cell 20.

According to the above, a layered body 100D illustrated in FIG. 3 can beproduced. The layered body 100D is a photovoltaic cell including twophotoelectric conversion cells, i.e., the GaInP cell 20 (1.9 eV) and theGaAs cell 30 (1.4 eV). That is to say, the layered body 100D is aphotovoltaic cell made of a compound semiconductor.

In the layered body 100D, the buffer layer 11 is an example of a firstbuffer layer, and the GaInP cell 20 and the GaAs cell 30 are examples ofa first photoelectric conversion cell. Furthermore, the layered body100B illustrated in FIG. 2B is an example of a first layered body, andthe Si substrate 10 is an example of a first silicon substrate. Thefirst photoelectric conversion cell is a multi-junction cell in whichtwo photoelectric conversion cells are stacked in series with each otherin the stack direction. The multi-junction cell may include three ormore photoelectric conversion cells that are stacked in series with eachother in the stack direction.

According to the photovoltaic cell manufacturing method according to thefirst embodiment described above, the layered body 100B (see FIG. 2A) isproduced by forming the buffer layer 11 on the Si substrate 10, andforming the contact layer 12, the GaInP cell 20, the tunnel junctionlayer 13, the GaAs cell 30, and the contact layer 14, on the bufferlayer 11.

Then, after removing the Si substrate 10 and the buffer layer 11 fromthe layered body 100B, the metal layer 17 is formed on the contact layer12, and the contact layer 12C is formed from the contact layer 12.

That is to say, by the photovoltaic cell manufacturing method accordingto the first embodiment, the buffer layer 11 for relaxing the differencein the lattice constant of silicon and the compound semiconductor layer,is formed on the inexpensive Si substrate 10, so that the GaInP cell 20,the tunnel junction layer 13, the GaAs cell 30, and the contact layer 14can be formed on the Si substrate 10.

Then, in the manufacturing process, the buffer layer 11 and the Sisubstrate 10 are removed from the layered body 100B (see FIG. 2A) by wetetching.

Accordingly, a photovoltaic cell (layered body 100D) made of a compoundsemiconductor can be manufactured at a low cost.

Furthermore, an anti-reflection film may be formed at the light incidentsurface (the part not covered by the contact layer 12C or the metallayer 17) on the top surface of the GaInP cell 20 of the photovoltaiccell (layered body 100D) illustrated in FIG. 3.

Note that in the above description, as the buffer layer 11, a layerhaving a two layer structure including a SiGe layer and a Ge layer isused, or a SiGe layer in which the ratio of Ge increases as the distancefrom the surface of the Si substrate 10 increases, is used.

However, the buffer layer 11 may be formed by directly depositing a Gelayer on the Si substrate 10 by a CVD method, causing a defect in the Gelayer by performing a thermal cycling process (TCA) so that a structurethat relaxes the difference in the lattice constant of Si and Ge isformed in the Ge layer, and performing a flattening process on thesurface of the Ge layer. On such a buffer layer 11, a wet etchingprocess may be performed by using an etching solution includinghydrofluoric acid, nitric acid, or acetic acid.

Furthermore, as a different method, after growing a GaAs layer on the Sisubstrate 10, TCA is performed, and then a strained super lattice layersuch as GaInAs/GaAs or GaInAs/GaPAs is grown, to be used as the bufferlayer 11.

The buffer layer 11 formed with such a strained super lattice layer issubjected to etching with the use of a mixed liquid including sulfuricacid (H₂SO₄), hydrogen peroxide (H₂O₂), and water (H₂O), and therefore aGaInP-layer is to be formed as an etching stop-layer at the boundarybetween the buffer layer 11 and the contact layer 12.

The GaInP-layer is not dissolved by a mixed liquid including sulfuricacid (H₂SO₄), hydrogen peroxide (H₂O₂), and water (H₂O), and thereforethe etching of the strained super lattice layer can be stopped at theGaInP-layer. Then, the GaInP-layer can be etched by a mixed liquid ofhydrochloric acid (HCl) and water (H₂O). The contact layer 12 (GaAs) isnot dissolved by a mixed liquid of hydrochloric acid (HCl) and water(H₂O), and therefore the GaInP-layer acting as the etching stop-layercan be selectively removed.

Furthermore, the buffer layer 11 may have a composition other than theabove. The buffer layer 11 may have any other composition, as long aslattice relaxation occurs according to the inconsistency in the latticeamong the Si substrate 10, the contact layer 12, the GaInP cell 20, thetunnel junction layer 13, the GaAs cell 30, and the contact layer 14. Bygenerating a dislocation in the buffer layer 11, a photovoltaic cellmade of a compound semiconductor having high quality with lessdislocation, can be formed on the buffer layer 11.

Furthermore, the support substrate 80 may be a glass substrate and a Sisubstrate, instead of a film made of plastic.

Furthermore, in the above description, the photovoltaic cell is adouble-junction photovoltaic cell including the GaInP cell 20 (1.9 eV)and the GaAs cell 30 (1.4 eV); however, a triple-junction photovoltaiccell formed by adding a cell having a band gap of 1.0 eV is even morepreferable in terms of further improving the quantum efficiency.

As an example of a cell having a band gap of 1.0 eV, a GaInNAs(Sb) cellor a GaPAs/GaInAs super lattice cell may be used.

Furthermore, in the first embodiment, a GaAs lattice matching materialis used; however, a Ge lattice matching material may be used. In thiscase, the lattice constant of the Ge lattice matching material isslightly higher than that of the GaAs lattice matching material, andtherefore the composition of the Ge lattice matching material is to beappropriately adjusted. Note that instead of the GaAs cell 30, it ispossible to use a cell made of GaInAs in which the composition of In isapproximately 1%. The outermost surface of the buffer layer 11 is Ge,which can be made more easily.

Furthermore, the photovoltaic cell manufactured by the photovoltaic cellmanufacturing method according to the first embodiment is not limited toa photovoltaic cell of a double-junction type or a triple-junction type.For example, a photovoltaic cell using a single-layered cell having acomposition of a (Al)GaInP cell, a GaAs cell, a GaInAs cell, aGaInNAs(Sb) cell, a GaInPAs cell, an InP cell, an AlInAs cell, etc., maybe manufactured. Furthermore, a photovoltaic cell of aquadruple-junction type or more may be manufactured.

According to the first embodiment described above, a photovoltaic cellmade of a compound semiconductor can be formed by using the Si substrate10, without using a compound semiconductor substrate, and ahigh-efficiency photovoltaic cell can be produced at a low cost.

Furthermore, the buffer layer 11 that performs lattice relaxation isremoved at the manufacturing stage, and therefore the buffer layer 11including many defects is not included in the final product of thephotovoltaic cell (layered body 100D), and a photovoltaic cell havinghigh efficiency and high reliability can be manufactured.

In the photovoltaic cell described in Non-patent Document 1, there is apossibility that the defects increase during operation and theefficiency deteriorates with time, and therefore the reliability maydecrease.

Meanwhile, in the photovoltaic cell manufactured by the manufacturingmethod according to the first embodiment, the buffer layer 11 forrelaxing the difference in the lattice constant does not remain in thefinal product of the photovoltaic cell, and therefore a photovoltaiccell having high reliability can be manufactured.

Furthermore, the Si substrate 10 is removed, and the photovoltaic cell(layered body 100D) is bonded together with the support substrate 80made of a plastic film, and therefore a flexible and light-weightphotovoltaic cell can be manufactured.

Note that (Al)GaInP encompasses both a composition including Al and acomposition not including Al, and therefore Al is expressed as (Al).That is to say, (Al)GaInP is an expression including both AlGaInP andGaInP.

Similarly, GaInNAs(Sb) is an expression including both GaInNAsSb andGaInNAs. Ga(In)As is an expression including both GaInAs and GaAs.Furthermore, (Al)GaInP(As) is an expression including AlGaInP, GaInPAs,and GaInP. Furthermore, GaIn(P)As is an expression including GaInPAs andGaInAs.

Note that the Si substrate 10 and the buffer layer 11 may be removed bya method other than wet etching as described above. For example, a liftoff method or a smart cut method may be performed, which includesselectively etching the AlAs by using an AlAs sacrifice layer, andseparating the substrate from the cell part.

The lift off method is described in, for example, Proceedings of the29th IEEE Photovoltaic Specialists Conference (2010) pp. 412-417. Thesmart cut method is often used for fabricating a SOI substrate, and isdescribed in, for example, Applied Physics Letter 92, 103503 (2008).

FIG. 4 illustrates a layered body 100E according to a modification ofthe first embodiment, in which the Si substrate 10 and the buffer layer11 are removed by a lift off method.

As illustrated in FIG. 4, in a case where a lift off method isperformed, in the layered body 100E, a sacrifice layer 110 constitutedby, for example, an AlAs layer, is formed between the buffer layer 11and the contact layer 12. Compared to the other materials, the AlAslayer has a significantly fast etching speed. Therefore, for example, byimpregnating the AlAs layer in a sulfuric acid medicinal solution, thesacrifice layer 110 is selectively etched form the side surfaces, andthe buffer layer 11 and the contact layer 12 are separated from eachother. Accordingly, the Si substrate 10 and the buffer layer 11 can beremoved.

FIGS. 5A and 5B illustrate a smart cut method according to amodification of the first embodiment.

As illustrated in FIG. 5A, in a case where a smart cut method isperformed, in a layered body 100F, a Si substrate 111 is includedinstead of the Si substrate 10 of the layered body 100B illustrated inFIG. 2A. In the smart cut method, before joining together the metallayer 15A and the metal layer 15B with the use of the bonding layer 16(see FIG. 1B), an ion implanter is used to implant approximately 3.5E16ions/cm² through 1E17 ions/cm² of hydrogen ions (H⁺), into the Sisubstrate 111. After joining the metal layer 15A and the metal layer 15Bwith the use of the bonding layer 16, heat treatment is performed atapproximately 400° C. through 600° C., so that peeling occurs at ahydrogen ion injection part 111A as illustrated in FIG. 5B, and the Sisubstrate 111 is separated into a thin Si layer 111B and a Si substrate111C.

Subsequently, the Si layer 111B and the buffer layer 11 are removed byperforming, for example, a wet etching process using an etching solutionincluding hydrofluoric acid, nitric acid, and acetic acid.

In this case, the Si substrate 111C may be reused. This method may beperformed in the same manner in the second through fourth embodimentsdescribed below.

Second Embodiment

A photovoltaic cell manufacturing method according to a secondembodiment involves using, as a support substrate, a Si substrate onwhich a photoelectric conversion cell made of a silicon semiconductor isformed. In this case, the band gap of the photoelectric conversion cellmade of a silicon semiconductor is 1.1 eV, and the band gap of thephotoelectric conversion cell made of a compound semiconductor is 1.7eV. Note that a GaInPAs cell is used as the photoelectric conversioncell made of a compound semiconductor.

FIGS. 6A through 7B illustrate a photovoltaic cell manufacturing methodaccording to a second embodiment. In FIGS. 6A through 7B, the elementsthat are the same as those described in the first embodiment are denotedby the same reference numerals, and descriptions thereof are omitted orsimplified.

First, as illustrated in FIG. 6A, the buffer layer 11, the contact layer12, a GaInPAs cell 220, a tunnel junction layer 213, and a bonding layer214A are sequentially formed on the Si substrate 10. In this case, alayered body 200 illustrated in FIG. 6A is made assuming that sunlightenters from the bottom side as viewed in FIG. 6A.

The GaInPAs cell 220 is a photoelectric conversion cell made of acompound semiconductor, including gallium (Ga), indium (In), phosphorus(P), and arsenic (As) as raw materials. The GaInPAs cell 220 includes ann-layer 221 and a p-layer 222. The GaInPAs cell 220 has substantiallythe same lattice constant as the Ge layer that is the topmost layer ofthe buffer layer 11 and the contact layer 12 (GaAs layer).

For example, the GaInPAs cell 220 is formed by a MOCVD method, bysequentially depositing the p-layer 221 and the p-layer 222 on thecontact layer 12.

As the dopant of the n-layer 221, for example, silicon (Si) or selenium(Se) may be used. Furthermore, as the dopant of the p-layer 222, forexample, zinc (Zn) or magnesium (Mg) may be used. In the secondembodiment, the composition of Ga, In, P, and As is adjusted, so thatthe band gap of the GaInPAs cell 220 becomes 1.7 eV.

Furthermore, the GaInPAs cell 220 has a lattice constant that issignificantly close to the lattice constant (approximately 5.65 Å) ofthe GaAs layer used as the contact layer 12. The composition of Ga, In,P, and As is adjusted so that the GaInPAs cell 220 can crystal-grow onthe contact layer 12. The lattice constant of GaInPAs used as theGaInPAs cell 220 is substantially the same value as the lattice constantof GaAs.

Note that between the n-layer 221 and the contact layer 12 (incidentside to the GaInPAs cell 220), a window layer having a wider gap thanthat of the GaInPAs cell 220 may be formed. Furthermore, above thep-layer 222 (between the p-layer 22 and the tunnel junction layer 213),a BSF (Back Surface Field) layer having a wider gap than that of theGaInPAs cell 220 may be formed.

The tunnel junction layer 213 is provided between the GaInPAs cell 220and the bonding layer 214A, and includes an n-layer and p-layer thathave been doped at a higher density than that of the n-layer 221 and thep-layer 222 of the GaInPAs cell 220 and an n-Si layer 232 and a p-Silayer 231 of a silicon cell 230 described below. The tunnel junctionlayer 213 is a junction layer, which is provided so that a current flowsbetween the p-layer 222 of the GaInPAs cell 220 and the n-Si layer 232of the silicon cell 230 (by tunnel junction).

For example, the tunnel junction layer 213 includes an AlGaAs layer(p-layer) and a GaInP-layer (n-layer), or a p-layer and an n-layerformed by a GaAs layer. For example, the tunnel junction layer 213 isformed by a MOCVD method, by sequentially depositing a p-layer and ann-layer on the surface of the GaInPAs cell 220 in the state order.

Note that the tunnel junction layer 213 has a lattice constant that issignificantly close to the lattice constant of the GaInPAs cell 220(approximately 5.65 Å). The composition is adjusted so that the tunneljunction layer 213 can crystal-grow on the GaInPAs cell 220.

The bonding layer 214A is mainly a layer to be deposited on the tunneljunction layer 213 for reducing the resistance between the GaInPAs cell220 and the silicon cell 230. For example, an n type gallium arsenide(n-GaAs) layer is used. The n-GaAs layer acting as the bonding layer214A may be formed on the tunnel junction layer 213 by, for example, aMOCVD method. As the dopant for making the layer a n type, for example,silicon (Si), selenium (Se), or tellurium (Te) may be used.

Note that the bonding layer 214A has a lattice constant that issignificantly close to the lattice constant of the tunnel junction layer213 and the GaInPAs cell 220 (approximately 5.65 Å). The composition isadjusted so that the bonding layer 214A can crystal-grow on the tunneljunction layer 213.

As described above, the layered body 200 illustrated in FIG. 6A isformed by sequentially depositing, on the Si substrate 10 and the bufferlayer 11, the contact layer 12 that constitutes the light incident side,the GaInPAs cell 220, the tunnel junction layer 213, and the bondinglayer 214A, in the stated order.

Furthermore, as illustrated in FIG. 6A, apart from the layered body 200,a bonding layer 214B is formed on the top surface of the silicon cell230. In this case, the silicon cell 230 includes a p type Si substrate(p-Si substrate) 231 and an n type silicon (n-Si) layer 232.

The n-Si layer 232 is formed by mixing impurities such as phosphorus (P)from one side of the p-Si layer 231 (top side as viewed in FIG. 6A) andcausing crystal-growth. The silicon cell 230 is a photoelectricconversion cell made of a silicon semiconductor.

Note that the n-Si layer 232 may be formed by implanting impurities suchas phosphorus (P) from one side of the p-Si layer 231 (top side asviewed in FIG. 6A).

The bonding layer 214B is mainly a layer to be deposited on the siliconcell 230 for reducing the resistance between the GaInPAs cell 220 andthe silicon cell 230. For example, an n type silicon-layer (n-Si layer)is used. The n-Si layer acting as the bonding layer 214B may be formedby mixing impurities such as phosphorus (P) from the top side of then-Si layer 232 and causing crystal-growth. The n-Si layer acting as thebonding layer 214B may be the same as the n-Si layer 232; however, then-Si layer acting as the bonding layer 214B preferably has a higherdensity than that of the n-Si layer 232.

Next, as illustrated in FIG. 6B, the layered body 200 is turned upsidedown in a state opposite to that illustrated in FIG. 6A, and the bondinglayer 214A of the layered body 200 and the bonding layer 214B on the topsurface of the silicon cell 230 are bonded together.

In this joining process, a cleaning process and a surface activationprocess are performed on the surfaces of the bonding layer 214A of thelayered body 200 and the bonding layer 214B on the top surface of thesilicon cell 230, to directly join together the bonding layer 214A andthe bonding layer 214B. Note that the surface activation process may beperformed by a nitrogen (N₂) plasma process, and the joining process maybe performed, for example, in a state where the bonding layer 214A andthe bonding layer 214B are heated to 150° C. in a vacuum atmosphere.

In this case, the layered body obtained by stacking the layered body 200and the silicon cell 230 as illustrated in FIG. 6B, is referred to as alayered body 200A.

Next, the Si substrate 10 and the buffer layer 11 are removed from thelayered body 200A by etching the Si substrate 10 and the buffer layer 11of the layered body 200A illustrated in FIG. 6B, so that a layered body200B illustrated in FIG. 7A is obtained. The layered body 200B is formedby removing the Si substrate 10 and the buffer layer 11 from the layeredbody 200A illustrated in FIG. 6B.

The etching of the Si substrate 10 and the buffer layer 11 is performed,for example, by a wet etching process using an etching solutionincluding hydrofluoric acid, nitric acid, or acetic acid, similar to thefirst embodiment.

Note that the above etching solution does not dissolve the contact layer12 (GaAs layer), the GaInPAs cell 220, the tunnel junction layer 213(AlGaAs/GaInP, or GaAs), the bonding layer 214A (GaAs layer), or thebonding layer 214B (GaAs layer). However, the above etching solutiondissolves the silicon cell 230.

Thus, the etching is to be performed by protecting, with a protectivelayer, the side surfaces of the Si substrate 10, the buffer layer 11,the contact layer 12, the GaInPAs cell 220, the tunnel junction layer213, the bonding layer 214A, the bonding layer 214B, and the siliconcell 230, and the surface of the silicon cell 230 (bottom surface inFIG. 6A), and then impregnating the layered body 200A in the etchingsolution. A protective layer will not be used when there is no need forprotection in the above etching process.

Furthermore, in this case, a description is given of an embodiment inwhich both the Si substrate 10 and the buffer layer 11 are removed byetching. However, by removing the buffer layer 11 with the use of asolution for selectively etching only the buffer layer 11, the Sisubstrate 10 may be removed from the layered body 200B. As such anetching solution, for example, a nitrohydrofluoric acid solution (amixed solution of hydrofluoric acid and nitric acid) may be used. Notethat water may be added to the nitrohydrofluoric acid solution to dilutethe solution.

Lastly, as illustrated in FIG. 7B, the metal layer 17 is formed on thetop surface of the contact layer 12 of the layered body 200B illustratedin FIG. 7A, and a metal layer 233 is formed on the bottom surface of thep-Si layer 231. Similar to the metal layer 17, the metal layer 233 canbe formed by vapor-depositing metal such as Au or Ag on the bottomsurface of the p-Si layer 231. The metal layer 233 is the bottomelectrode of the photovoltaic cell (layered body 200C) according to thesecond embodiment.

Furthermore, the contact layer 12C illustrated in FIG. 7B is the same asthe contact layer 12C of the first embodiment, which is formed byforming the metal layer 17 (see FIG. 7B) on the top surface of thecontact layer 12 illustrated in FIG. 7A, and then using this metal layer17 as a mask in removing parts of the contact layer 12 (see FIG. 7A)other than the part positioned immediately below the metal layer 17.

The contact layer 12C may be made by, for example, using a mixed liquidincluding sulfuric acid (H₂SO₄), hydrogen peroxide (H₂O₂), and water(H₂O), as the wet etching solution. The mixed liquid including sulfuricacid (H₂SO₄), hydrogen peroxide (H₂O₂), and water (H₂O) does notdissolve the GaInP in the GaInPAs cell 220, and therefore the wetetching process can be stopped at the GaInPAs cell 220.

According to the above, a layered body 200C illustrated in FIG. 7B canbe produced. The layered body 200C is a photovoltaic cell including twophotoelectric conversion cells, i.e., the GaInPAs cell 220 (1.7 eV) andthe silicon cell 230 (1.1 eV). That is to say, the layered body 200C isa double-junction photovoltaic cell including a photoelectric conversioncell made of a compound semiconductor and a photoelectric conversioncell made of a silicon semiconductor.

In the layered body 200C, the buffer layer 11 is an example of a firstbuffer layer, and the GaInPAs cell 220 is an example of a firstphotoelectric conversion cell. Furthermore, the layered body 200Aillustrated in FIG. 6B is an example of a first layered body, and the Sisubstrate 10 is an example of a first silicon substrate. Furthermore,the silicon cell 230 is an example of a photoelectric conversion cellmade of a silicon semiconductor.

According to the photovoltaic cell manufacturing method according to thesecond embodiment described above, the GaInPAs cell 220 (1.7 eV) havinga wide band gap is disposed closer to the light incident side, than thesilicon cell 230 (1.1 eV). This is to absorb the short wavelength lightat the GaInPAs cell 220 on the light incident side, and to absorb thelight having a relatively long wavelength that has been transmittedthrough the GaInPAs cell 220, at the silicon cell 230.

The energy conversion efficiency of a double-junction photovoltaic cellincluding 1.7 eV/1.1 eV is estimated to be about the same level as thatof a triple-junction photovoltaic cell including 1.9 eV/1.4 eV/0.67 eV(see, for example, The Japan Society of Applied Physics Autumnproceedings, 2010, 15p-NC-4).

In consideration of the carrier at the junction interface, the measuresfor loss of light, and the ease of current matching, the double-junctionphotovoltaic cell is more practical than the triple-junctionphotovoltaic cell.

According to the photovoltaic cell manufacturing method according to thesecond embodiment described above, the layered body 200 (see FIG. 6A) ismade by forming the buffer layer 11 on the inexpensive Si substrate 10,and forming the contact layer 12, the GaInPAs cell 220, the tunneljunction layer 213, and the bonding layer 214A, on the buffer layer 11.

Furthermore, the bonding layer 214A of the layered body 200 and thebonding layer 214B formed on the top surface of the silicon cell 230 arebonded together, so that the layered body 200A (see FIG. 6B) includingthe GaInPAs cell 220 and the silicon cell 230 is made.

Then, after removing the Si substrate 10 and the buffer layer 11 fromthe layered body 200A and obtaining the layered body 200B (see FIG. 7A),the metal layer 17 is formed on the contact layer 12 of the layered body200B. Furthermore, the contact layer 12C is formed from the contactlayer 12, thereby fabricating the photovoltaic cell according to thesecond embodiment (layered body 200C, see FIG. 7B).

That is to say, by the photovoltaic cell manufacturing method accordingto the second embodiment, a photovoltaic cell including a photoelectricconversion cell made of a compound semiconductor can be produced withoutusing a compound semiconductor substrate. Therefore, by the photovoltaiccell manufacturing method according to the second embodiment, ahigh-efficiency photovoltaic cell can be produced at a low cost.

Furthermore, the buffer layer 11, which realizes lattice relaxation forcancelling out the inconsistency in the lattice constant between the Sisubstrate 10 and the photovoltaic cell made of a compound semiconductor,is removed, and is not included in the final product of the photovoltaiccell (layered body 200C). Therefore, a photovoltaic cell (layered body200C) having high reliability can be provided.

In the photovoltaic cell manufactured by the manufacturing methodaccording to the second embodiment, the buffer layer 11 that relaxes thedifference in the lattice constant does not remain in the final productof the photovoltaic cell, and therefore a photovoltaic cell havinghigher reliability than the photovoltaic cell described in Non-patentDocument 1 can be manufactured.

Furthermore, according to the photovoltaic cell manufacturing methodaccording to the second embodiment, a photovoltaic cell (layered body200C) with which high efficiency can be attained by a double-junctionstructure, can be easily manufactured.

Note that an anti-reflection film may be formed at the light incidentsurface (the part not covered by the contact layer 12C or the metallayer 17) on the top surface of the GaInPAs cell 220 of the photovoltaiccell (layered body 200C) illustrated in FIG. 7B.

Furthermore, the photovoltaic cell according to the second embodimentmay be modified as illustrated in FIG. 8.

FIG. 8 is a cross-sectional view of a photovoltaic cell according to amodification of the second embodiment.

A photovoltaic cell 200D illustrated in FIG. 8 does not include thetunnel junction layer 213 illustrated in FIG. 7B, and the bonding layer214A and the bonding layer 214B are mechanically connected by a fixingmember 280. As the fixing member 280, for example, a Palladium (Pd)Nanoparticle Array may be used.

The palladium nanoparticle array is for causing conductive nanoparticlesto be self-arranged on the junction interface, with the use of theseparation arrangement of a block copolymer. Nano arrangements of Pd,Au, Pt, Ag, etc., are possible. A diluted solution of a block copolymeris spin-coated, the block copolymer is caused to be self-arranged, andthe block copolymer is exposed to an aqueous solution including metalions such as Pd²+ (palladium ions), so that metal ions are selectivelyformed in the block copolymer. Then, Ar (argon) plasma is radiated, sothat the block copolymer template is removed, and a nanoparticle arraywhich is self-arranged, is formed. Light is transmitted through partswithout nanoparticles. By using a palladium nanoparticle array, lightthat has been transmitted through the GaInPAs cell 220 can beefficiently guided to the silicon cell 230.

In a state where a palladium nanoparticle array is formed on the bondinglayer 214A or the bonding layer 214B, the bonding layer 214A and thebonding layer 214B are bonded together, so that a compound semiconductorphotovoltaic cell is produced.

The fixing member 280 is an example of a fixing unit. As describedabove, the bonding method of mechanically superposing two layered bodieswith the use of the fixing member 280 is referred to as mechanicalstacking.

Note that the fixing member 280 is not limited to a palladiumnanoparticle array; the fixing member 280 may be a nanoparticle arrayincluding other metals (for example, Au (gold), or other mechanicalmeans.

As described above, in the photovoltaic cell according to themodification of the second embodiment, the bonding layer 214A and thebonding layer 214B are bonded by the fixing member 280, and thereforethere is no need to provide the tunnel junction layer 213 (see FIG. 7B)between the GaInPAs cell 220 and the bonding layer 214A, whereas theGaInPAs cell 220 is directly bonded on top of the bonding layer 214A.

As described above, a layered body including the silicon cell 230 andthe layered body including the GaInPAs cell 220 may be connected bymechanical stacking.

According to the second embodiment, the band gap of the photoelectricconversion cell made of the compound semiconductor is 1.7 eV asdescribed above. However, the band gap of the photoelectric conversioncell made of the compound semiconductor may be 1.4 eV through 1.9 eV.

Third Embodiment

A photovoltaic cell manufacturing method according to a third embodimentis a method of manufacturing a quadruple-junction photovoltaic cellincluding four photoelectric conversion cells made of a compoundsemiconductor. In the photovoltaic cell according to the thirdembodiment, two of the four photoelectric conversion cells have adifferent lattice constant from that of the other two of the fourphotoelectric conversion cells. Thus, the quadruple-junctionphotovoltaic cell is manufactured by fabricating two photoelectricconversion cells on each of two Si substrates, and then joining the twoSi substrates.

FIGS. 9A through 13 illustrate a photovoltaic cell manufacturing methodaccording to the third embodiment. In FIGS. 9A through 13, the elementsthat are the same as those described in the first embodiment are denotedby the same reference numerals, and descriptions thereof are omitted orsimplified.

First, layered bodies 300A and 300B illustrated in FIG. 9 are made.

The layered body 300A is made by sequentially forming a buffer layer311A, a contact layer 312A, a GaInAs cell 340, a tunnel junction layer313A, a GaInPAs cell 350, and a bonding layer 314A, on a Si substrate10A.

Furthermore, the layered body 300B is made by sequentially forming abuffer layer 11B, a contact layer 12B, a GaInP cell 20, a tunneljunction layer 13, a GaAs cell 30, a tunnel junction layer 313B, and abonding layer 314B, on a Si substrate 10B.

The layered body 300A illustrated in FIG. 9 is made assuming thatsunlight enters from the top side as viewed in FIG. 9, and the layeredbody 300B illustrated in FIG. 9 is made assuming that sunlight entersfrom the bottom side as viewed in FIG. 9.

First, a description is given of the layered body 300A. The Si substrate10A included in the layered body 300A is the same as the Si substrate 10of the first embodiment.

The buffer layer 311A is formed by directly depositing a Ge layer on theSi substrate 10A by a CVD method, and causing a defect in the Ge layerby performing a thermal cycling process (TCA) so that a structure thatrelaxes the difference in the lattice constant of Si and Ge is formed inthe Ge layer. Subsequently, a flattening process is performed on thesurface of the Ge layer.

Then, on the Ge layer on which the flattening layer has been performed,a GaAs layer is formed by a MOCVD method. Furthermore, on the GaAslayer, a composition sloped layer made of AlInAs is formed by a MOCVDmethod, so that further lattice relaxation is realized.

Specifically, as the composition sloped layer made of AlInAs, thecomposition ratio of In in the layer is gradually increased (in a slopedmanner), from AlAs having a lattice constant that is substantially equalto that of GaAs, to AlInAs composition having a lattice constant thatslightly exceeds that of InP. Then, the composition ratio of In isslightly reduced just before the topmost layer of the AlInAs compositionsloped layer, so that the lattice constant of the AlInAs compositionsloped layer is reduced to a lattice constant that is substantiallyequal to that of InP. Then, as the topmost layer of the compositionsloped layer made of AlInAs, a layer having a fixed composition ratio ofIn is formed, by which approximately the same lattice constant as thatof InP (approximately 5.87 Å) can be obtained.

That is to say, as the buffer layer 311A, a layer including a Ge layer,a GaAs layer, and a composition sloped layer made of AlInAs, is formedon the Si substrate 10A.

Furthermore, the contact layer 312A is formed by depositing a GaInAslayer by a MOCVD method on the buffer layer 311A. Note that the GaInAslayer formed as the contact layer 312A has a lattice constant that issignificantly close to that of InP, and the composition of Ga, In, As isadjusted so that the GaInAs layer can crystal-grow on the buffer layer311A.

The GaInAs cell 340 is a photoelectric conversion cell made of acompound semiconductor including gallium (Ga), indium (In), and arsenic(As) as raw materials, and the GaInAs cell 340 includes an n-layer 341and a p-layer 342. For example, the GaInAs cell 340 is formed bysequentially depositing the p-layer 342 and the n-layer 341 on thecontact layer 312A by a MOCVD method.

As the dopant of the n-layer 341, for example, silicon (Si) or selenium(Se) may be used. Furthermore, as the dopant of the p-layer 342, forexample, zinc (Zn) or magnesium (Mg) may be used. In the thirdembodiment, the composition of Ga, In, As is adjusted, so that the bandgap of the GaInAs cell 340 becomes 0.7 eV.

Note that the GaInAs cell 340 has a lattice constant that issignificantly close to that of InP, and the composition of Ga, In, As isadjusted so that the GaInAs cell 340 can crystal-grow on the contactlayer 312A.

Note that between the n-layer 341 and the tunnel junction layer 313A(incident side to the GaInAs cell 340), a window layer having a widergap than that of the GaInAs cell 340 may be formed. Furthermore, belowthe p-layer 342 (between the p-layer 342 and the contact layer 312A), aBSF (Back Surface Field) layer having a wider gap than that of theGaInAs cell 340 may be formed.

The tunnel junction layer 313A is provided between the GaInAs cell 340and the GaInPAs cell 350, and includes an n-layer and a p-layer that aredoped by a higher density than that of the n-layer 341 and the p-layer342 of the GaInAs cell 340, and a n-layer 351 and the p-layer 352 of theGaInPAs cell 350. The tunnel junction layer 313A is a junction layerwhich is provided so that a current flows between the n-layer 341 of theGaInAs cell 340 and the p-layer 352 of the GaInPAs cell 350 (by tunneljunction).

For example, the tunnel junction layer 313A is formed by depositing then-layer and the p-layer of the AlGaInAs layer in the stated order on thesurface of the GaInAs cell 340 by a MOCVD method.

Note that the tunnel junction layer 313A has a lattice constant that issignificantly close to that of InP, and the composition of Al, Ga, In,As is adjusted so that the tunnel junction layer 313A can crystal-growon the GaInAs cell 340.

The GaInPAs cell 350 is a photoelectric conversion cell made of acompound semiconductor, including gallium (Ga), indium (In), phosphorus(P), and arsenic (As) as raw materials. The GaInPAs cell 350 includesthe n-layer 351 and the p-layer 352. For example, the GaInPAs cell 350is formed by a MOCVD method, by sequentially depositing the p-layer 352and the n-layer 351 on the tunnel junction layer 313A.

As the dopant of the n-layer 351, for example, silicon (Si) or selenium(Se) may be used. Furthermore, as the dopant of the p-layer 352, forexample, zinc (Zn) or magnesium (Mg) may be used. In the thirdembodiment, the composition of Ga, In, P, and As is adjusted, so thatthe band gap of the GaInPAs cell 350 becomes 1.0 eV.

Note that the GaInPAs cell 350 has a lattice constant that issignificantly close to that of InP, and the composition of Ga, In, P,and As is adjusted so that the GaInPAs cell 350 can crystal-grow on thetunnel junction layer 313A.

Note that between the n-layer 351 and the bonding layer 314A (incidentside to the GaInPAs cell 350), a window layer having a wider gap thanthat of the GaInPAs cell 350 may be formed. Furthermore, below thep-layer 352 (between the p-layer 352 and the tunnel junction layer313A), a BSF (Back Surface Field) layer having a wider gap than that ofthe GaInPAs cell 350 may be formed.

The bonding layer 314A is mainly a layer to be bonded with the bondinglayer 314B to connect the layered body 300A and the layered body 300B,and a layer to be deposited on the GaInPAs cell 350 for reducing theresistance between the GaInPAs cell 350 of the layered body 300A and theGaAs cell 30 of the layered body 300B.

For example, as the bonding layer 314A, a thin n-InP-layer that has beendoped at high density, is used. For example, the n-InP-layer used as thebonding layer 314A may be formed on the GaInPAs cell 350 by an MOCVDmethod.

Note that the bonding layer 314A has a lattice constant that issignificantly close to the lattice constant of InP. The composition isadjusted so that the bonding layer 314A can crystal-grow on the GaInPAscell 350.

As described above, the layered body 300A illustrated in FIG. 9 isformed by sequentially depositing, on the Si substrate 10A and thebuffer layer 311A, the contact layer 312A that is at the far side withrespect to the light incident direction, the GaInAs cell 340, the tunneljunction layer 313A, the GaInPAs cell 350, and the bonding layer 314A,in the stated order.

Accordingly, the GaInPAs cell 350 (1.0 eV) having a wide band gap isproduced closer to the light incident side, than the GaInAs cell 340(0.7 eV).

Note that in the third embodiment, the Si substrate 10A and the bufferlayer 311A are examples of a first silicon substrate and a first bufferlayer, respectively. The GaInAs cell 340 and the GaInPAs cell 350 areexamples of a first photoelectric conversion cell, and are also examplesof a cell formed with an InP lattice matching material. Furthermore, thebonding layer 314A is an example of a first joining layer.

Next, a description is given of the layered body 300B. In the layeredbody 300B, the Si substrate 10B, the buffer layer 11B, the contact layer12B, the GaInP cell 20, the tunnel junction layer 13, and the GaAs cell30, are the same as the Si substrate 10, the buffer layer 11, thecontact layer 12, the GaInP cell 20, the tunnel junction layer 13, andthe GaAs cell 30 of the first embodiment, respectively.

In the layered body 300B, the tunnel junction layer 313B and the bondinglayer 314B are deposited on the GaAs cell 30.

The tunnel junction layer 313B is provided between the GaAs cell 30 andthe bonding layer 314B, and includes an n-layer and a p-layer that aredoped by a higher density than that of the n-layer 31 and the p-layer 32of the GaAs cell 30. The tunnel junction layer 313B is a junction layerwhich is provided so that a current flows between the p-layer 32 of theGaAs cell 30 and the bonding layer 314B (by tunnel junction).

For example, the tunnel junction layer 313B includes a p-layer and ann-layer according to a GaAs layer. The tunnel junction layer 313B isformed by depositing the p-layer and the n-layer in the stated order onthe surface of the GaAs cell 30 by a MOCVD method.

Note that the tunnel junction layer 313B has a lattice constant that issignificantly close to that of the GaAs layer (approximately 5.65 Å),and the composition is adjusted so that the tunnel junction layer 313Bcan crystal-grow on the GaAs cell 30.

The bonding layer 314B is mainly a layer to be bonded with the bondinglayer 314A to connect the layered body 300A and the layered body 300B,and a layer to be deposited on the tunnel junction layer 313B forreducing the resistance between the GaAs cell 30 and the tunnel junctionlayer 313B of the layered body 300B and the GaInPAs cell 350 of thelayered body 300A.

For example, as the bonding layer 314B, an n type gallium arsenide(n-GaAs) layer is used. For example, the GaAs layer used as the bondinglayer 314B may be formed on the tunnel junction layer 313B by a MOCVDmethod.

Note that the bonding layer 314B has a lattice constant that issignificantly close to that of the GaAs layer (approximately 5.65 Å),and the composition is adjusted so that the bonding layer 314B cancrystal-grow on the tunnel junction layer 313B.

Note that between the n-layer 21 of the GaInP cell 20 and the contactlayer 12B (incident side to the GaInP cell 20), a window layer having awider gap than that of the GaInP cell 20 may be formed. Furthermore,above the p-layer 22 (between the p-layer 22 and the tunnel junctionlayer 13), a BSF (Back Surface Field) layer having a wider gap than thatof the GaInP cell 20 may be formed.

Note that between the n-layer 31 of the GaAs cell 30 and the tunneljunction layer 13 (incident side to the GaAs cell 30), a window layerhaving a wider gap than that of the GaAs cell 30 may be formed.Furthermore, above the p-layer 32 (between the p-layer 32 and the tunneljunction layer 313B), a BSF (Back Surface Field) layer having a widergap than that of the GaAs cell 30 may be formed.

In the third embodiment, the Si substrate 10B and the buffer layer 11Bare examples of a second silicon substrate and a second buffer layer,respectively. The GaInP cell 20 and the GaAs cell 30 are examples of asecond photoelectric conversion cell, and are also examples of a cellformed with a GaAs lattice matching material. Furthermore, the bondinglayer 314B is an example of a second joining layer.

Next, as illustrated in FIG. 10, the layered body 300A is turned upsidedown, and the bonding layer 314A and the bonding layer 314B are bondedtogether, to directly join the layered body 300A with the layered body300B. A layered body that is obtained by connecting the layered body300A and the layered body 300B as illustrated in FIG. 10, is referred toas a layered body 300C. The layered body 300C is an example of a secondlayered body.

In this joining process, a cleaning process and a surface activationprocess are performed on the surfaces of the bonding layer 314A of thelayered body 300A and the bonding layer 314B of the layered body 300B,to directly join together the bonding layer 314A and the bonding layer314B. Note that the surface activation process may be performed by anitrogen (N₂) plasma process, and the joining process may be performed,for example, in a state where the bonding layer 314A and the bondinglayer 314B are heated to 150° C. in a vacuum.

Next, the Si substrate 10A and the buffer layer 311A are removed fromthe layered body 300C by etching the Si substrate 10A and the bufferlayer 311A of the layered body 300C illustrated in FIG. 10, so that alayered body 300D illustrated in FIG. 11A is obtained. The layered body300D is formed by removing the Si substrate 10A and the buffer layer311A from the layered body 300C illustrated in FIG. 10.

The etching of the Si substrate 10A and the buffer layer 311A isperformed by, for example, etching the Ge layers in the Si substrate 10Aand the buffer layer 311A, with a mixed solution (HF:HNO₃:CH₃COOH mixedsolution) including hydrofluoric acid (HF), nitric acid (HNO₃), andacetic acid (CH₃COOH). Furthermore, the GaAs layer and the AlInAscomposition sloped layer in the buffer layer 311A are to be etched witha mixed solution (H₂SO₄:H₂O₂:H₂O mixed solution) including sulfuric acid(H₂SO₄), hydrogen peroxide (H₂O₂), and water (H₂O).

In this case, the mixed solution including sulfuric acid (H₂SO₄),hydrogen peroxide (H₂O₂), and water (H₂O) dissolves the GaInAs in thecontact layer 312A, and therefore by providing an etching stop-layersuch as a GaInP-layer between the buffer layer 311A and the contactlayer 312A, the wet etching process can be stopped.

Note that the etching is to be performed by protecting, with aprotective layer, the portions other than the surface of the Sisubstrate 10A of the layered body 300C (top surface as viewed in FIG.10), and then impregnating the layered body 300C in the etchingsolution.

That is to say, the side surfaces of the Si substrate 10A, the bufferlayer 311A, the contact layer 312A, the GaInAs cell 340, the tunneljunction layer 313A, the GaInPAs cell 350, the bonding layer 314A, thebonding layer 314B, the tunnel junction layer 313B, the GaAs cell 30,the tunnel junction layer 13, the GaInP cell 20, the contact layer 12B,the buffer layer 11B, the Si substrate 10B, and the surface of the Sisubstrate 10B (bottom surface as viewed in FIG. 10) are to be protectedby a protective layer, and then the layered body 300C is to beimpregnated in the etching solution. A protective layer will not be usedwhen there is no need for protection in the above etching process.

Furthermore, in this case, a description is given of an embodiment inwhich both the Si substrate 10A and the buffer layer 311A are removed byetching. However, by removing the buffer layer 311A with the use of asolution for selectively etching only the buffer layer 311A, the Sisubstrate 10A may be removed from the layered body 300C. As such anetching solution, for example, a nitrohydrofluoric acid solution (amixed solution of hydrofluoric acid and nitric acid) may be used. Notethat water may be added to the nitrohydrofluoric acid solution to dilutethe solution.

Next, as illustrated in FIG. 11B, a support substrate 80 is prepared,and a metal layer 315A is formed on the contact layer 312A of thelayered body 300D (see FIG. 11A), and a metal layer 15B is formed on thesupport substrate 80.

The metal layers 315A, 15B are thin films made of a metal such as gold(Au) or silver (Ag), and may be formed by a vapor deposition method or asputtering method. Furthermore, the support substrate 80 may be, forexample, a film made of plastic.

Note that a layered body obtained by forming the metal layer 315A on thecontact layer 312A of the layered body 300D (see FIG. 11A) asillustrated in FIG. 11B, is referred to as a layered body 300E. Thelayered body 300E is an example of a third layered body.

Next, as illustrated in FIG. 12A, the layered body 300E (see FIG. 11B)is turned upside down, and a bonding layer 316 is used to join the metallayer 315A of the layered body 300E with the metal layer 15B formed onthe surface of the support substrate 80.

The bonding layer 316 is the same as the bonding layer 16 of the firstembodiment, and is formed by applying, on the surface of the metal layer315A or the metal layer 15B, a joining material such as a conductiveepoxy agent formed, for example, by including silver (Ag) nanoparticlesin epoxy resin. The bonding layer 316 is formed by, for example,applying the joining material on the metal layer 315A or the metal layer15B by screen printing.

A layered body 300F as illustrated in FIG. 12A is formed by joiningtogether the metal layer 315A and the metal layer 15B by using thebonding layer 316 as described above. In this case, the bonding layer316 is heated so that the metal layer 315A and the metal layer 15B arefused together by the bonding layer 316. The layered body 300F includesthe layered body 300E (see FIG. 11B), the bonding layer 316, the metallayer 15B, and the support substrate 80.

Note that the metal layer 315A, the bonding layer 316, and the metallayer 15B are used as the bottom electrode (backside electrode) of thephotovoltaic cell.

Next, the Si substrate 10B and the buffer layer 11B are removed from thelayered body 300F by etching the Si substrate 10B and the buffer layer11B of the layered body 300F illustrated in FIG. 12A. Accordingly, alayered body 300G as illustrated in FIG. 12B is formed. The layered body300G is obtained by removing the Si substrate 10B and the buffer layer11B from the layered body 300F illustrated in FIG. 12A.

The etching of the buffer layer 11B may be performed in the same manneras etching the buffer layer 11 of the first embodiment, for example, bya wet etching process using an etching solution including hydrofluoricacid, nitric acid, or acetic acid.

Note that the etching is to be performed by protecting, with aprotective layer, the portions other than the surface of the Sisubstrate 10B of the layered body 300F (top surface as viewed in FIG.12A), and then impregnating the layered body 300F in the etchingsolution.

That is to say, the side surfaces of the Si substrate 10B, the bufferlayer 11B, the contact layer 12B, the GaInP cell 20, the tunnel junctionlayer 13, the GaAs cell 30, the tunnel junction layer 313B, the bondinglayer 314B, the bonding layer 314A, the GaInPAs cell 350, the tunneljunction layer 313A, the GaInAs cell 340, the contact layer 312A, themetal layer 315A, the bonding layer 316, the metal layer 15B, and thesupport substrate 80, and the surface of the support substrate 80(bottom surface as viewed in FIG. 12A) are to be protected by aprotective layer, and then the layered body 300F is to be impregnated inthe etching solution. A protective layer will not be used when there isno need for protection in the above etching process.

Furthermore, in this case, a description is given of an embodiment inwhich both the Si substrate 10B and the buffer layer 11B are removed byetching. However, by removing the buffer layer 11B with the use of asolution for selectively etching only the buffer layer 11B, the Sisubstrate 10B may be removed from the layered body 300F. As such anetching solution, for example, a nitrohydrofluoric acid solution (amixed solution of hydrofluoric acid and nitric acid) may be used. Notethat water may be added to the nitrohydrofluoric acid solution to dilutethe solution.

Lastly, as illustrated in FIG. 13, a metal layer 17 is formed on the topsurface of the contact layer 12B of the layered body 300G (see FIG.12B). The metal layer 17 is formed on part of the top surface of thecontact layer 12B (see FIG. 12B). This is because the metal layer 17becomes the top electrode of the photovoltaic cell.

The metal layer 17 may be formed by, for example, a lift off method. Themetal layer 17 is formed by forming a resist on the top surface of thecontact layer 12B illustrated in FIG. 12B, in areas other than the areawhere the metal layer 17 illustrated in FIG. 13 is to be formed,vapor-depositing metal such as Au or Ag above the resist, and thenremoving the resist.

Note that a contact layer 12C illustrated in FIG. 3 is formed by formingthe metal layer 17 (see FIG. 13) on the top surface of the contact layer12B illustrated in FIG. 12B, and then using this metal layer 17 as amask in removing parts of the contact layer 12B (see FIG. 12B) otherthan the part positioned immediately below the metal layer 17.

The contact layer 12C may be made by, for example, using a mixed liquidincluding sulfuric acid (H₂SO₄), hydrogen peroxide (H₂O₂), and water(H₂O), as the wet etching solution.

According to the above, a layered body 300H illustrated in FIG. 13 canbe made. The layered body 300H is a photovoltaic cell including fourphotoelectric conversion cells from the light incident direction, i.e.,the GaInP cell 20 (1.9 eV), the GaAs cell 30 (1.4 eV), the GaInPAs cell350 (1.0 eV), and the GaInAs cell 340 (0.7 eV). That is to say, thelayered body 300H is a photovoltaic cell made of a compoundsemiconductor.

A document (Proceedings of the 28th IEEE Photovoltaic SpecialistsConference (2009) pp. 1090-1093.) describes that in a quadruple-junctionphotovoltaic cell, the balance of band gaps of approximately 1.9 eV/1.4eV/1.0 eV/0.7 eV is preferable.

Furthermore, a document (Applied physics, volume 79, no. 5, 2010, P.436) describes that a combination of 1.9 eV/1.4 eV/1.0 eV and acombination of 1.7 eV/1.2 eV/0.67 eV are more preferable than thepresent triple-junction cell (1.9 eV/1.4 eV/0.67 eV) described in thedocument.

Incidentally, it is difficult to realize a combination of band gaps asdescribed above, with a single substrate. This is because the abovequadruple-junction photovoltaic cell and the triple-junctionphotovoltaic cell include photoelectric conversion cells havingdifferent lattice constants.

Meanwhile, by the photovoltaic cell manufacturing method according tothe third embodiment, photoelectric conversion cells having differentlattice constants (340 and 350; and 20 and 30) are formed on separate Sisubstrates 10A and 10B. Furthermore, the photoelectric conversion cellshaving different lattice constants (340 and 350; and 20 and 30) formedon separate Si substrates 10A and 10B, are bonded together by a directbonding method, to manufacture a photovoltaic cell.

Thus, according to the third embodiment, a photovoltaic cell includingphotoelectric conversion cells of different lattice constants, can beeasily manufactured.

Note that in the third embodiment, a description is given of anembodiment of manufacturing a photovoltaic cell (layered body 300H)including a combination of band gaps of 1.9 eV/1.4 eV/1.0 eV/0.7 eV.

However, the combination of band gaps is not so limited. By changing thecomposition of materials in each photoelectric conversion cell, thebalance of the band gap can be changed. Therefore, by changing thecomposition of materials in each photoelectric conversion cell, thebalance of the band gaps can be optimized.

For example, a document (Progress in Photovoltaics 10, 2002, pp.323-329) describes that in a quadruple-junction photovoltaic cell, thecombination of 2.1 eV/1.5 eV/1.1 eV/0.8 eV is preferable.

In the photovoltaic cell (layered body 300H) according to the thirdembodiment, an AlGaInP cell is formed by adding Al to the GaInP cell 20,so that the band gap can be adjusted to 2.1 eV.

Furthermore, a GaInPAs cell is formed by adding In and P to the GaAscell 30, so that the band gap can be adjusted to 1.5 eV.

Furthermore, by adjusting the composition of the GaInPAs cell 350, theband gap can be adjusted to 1.1 eV.

Furthermore, a GaInPAs cell is formed by adding P to the GaInAs cell340, so that the band gap can be adjusted to 0.8 eV.

The energy conversion efficiency of the quadruple-junction photovoltaiccell (layered body 300H) having a combination of 1.9 eV/1.4 eV/1.0eV/0.7 eV, according to an embodiment, is higher than that of atriple-junction photovoltaic cell having a combination of 1.9 eV/1.4eV/0.67 eV.

Thus, according to the third embodiment, a high-efficiency compoundsemiconductor photovoltaic cell can be produced at a low cost, withoutusing a compound semiconductor substrate.

Furthermore, in the third embodiment, a description is given of a caseof joining together a GaAs lattice matching material with an InP latticematching material; however, a Ge lattice matching material may be usedinstead of a GaAs lattice matching material. In this case, the latticeconstant of the Ge lattice matching material is slightly higher than thelattice constant of the GaAs lattice matching material; and thereforethe composition of the Ge lattice matching material is to beappropriately adjusted. Note that instead of the GaAs cell 30, a cellmade of GaInAs having an In composition of approximately 1% may be used.The outermost surface of the buffer layer 11 is Ge, which can be mademore easily.

Furthermore, the buffer layers 11B, 311A, which are for realizinglattice relaxation for eliminating an inconsistency in the latticeconstant of the Si substrate 10 and the photoelectric conversion cellsmade of compound semiconductor (20, 30, 340, 350), include many defects(lattice defects). Therefore, if the buffer layers 11B, 311A areincluded in the final product, a recombination center may be formed,which may deteriorate the efficiency.

However, the buffer layers 11B, 311A are removed in a mid-procedure ofthe photovoltaic cell manufacturing method according to the thirdembodiment, and are not included in the final product (layered body300H).

Therefore, by the photovoltaic cell manufacturing method according tothe third embodiment, a photovoltaic cell (layered body 300H) havinghigh reliability can be manufactured.

In the photovoltaic cell manufactured by the manufacturing methodaccording to the third embodiment, the buffer layers 11B, 311A forrelaxing the difference in the lattice constant do not remain in thefinal product of the photovoltaic cell, and therefore it is possible tomanufacture a photovoltaic cell having higher reliability than thephotovoltaic cell described in Non-patent Document 1.

Furthermore, according to the third embodiment, the Si substrates 10A,10B are removed, and photoelectric conversion cells (20, 30, 340, 350)made of a compound semiconductor are bonded together with the supportsubstrate 80 made of a plastic film, and therefore a flexible andlight-weight photovoltaic cell can be manufactured.

In the above description, the GaInAs cell 340 and the GaInPAs cell 350are formed as examples of cells formed with an InP lattice matchingmaterial. That is to say, the InP lattice matching material can beexpressed as GaIn(P)As.

Furthermore, in the above description, the GaInP cell 20 and the GaAscell 30 are formed as examples of cells formed with a GaAs matchingmaterial. Instead of the GaInP cell 20, a cell formed of a materialexpressed by (Al)GaInP(As) may be used.

Note that an anti-reflection film may be formed at the light incidentsurface (the part not covered by the contact layer 312A or the metallayer 17) on the top surface of the GaInP cell 20 of the photovoltaiccell (layered body 100D) illustrated in FIG. 13.

Furthermore, similar to the modification (see FIG. 8) of the secondembodiment, the bonding layer 314A and the bonding layer 314B may beconnected by a fixing member. In this case, the tunnel junction layer313B is unnecessary.

Fourth Embodiment

A photovoltaic cell manufacturing method according to a fourthembodiment is for manufacturing a triple-junction photovoltaic cellincluding three photoelectric conversion cells of an AlInAs cell (1.9eV)/a GaInPAs cell (1.3 eV)/a GaInAs cell (0.9 eV).

FIGS. 14A through 16 illustrate a photovoltaic cell manufacturing methodaccording to a fourth embodiment.

First, as illustrated in FIG. 14A, a buffer layer 411, a contact layer412, an AlInAs cell 420, a tunnel junction layer 413A, a GaInPAs cell430, a tunnel junction layer 413B, a GaInAs cell 440, and a contactlayer 414 are sequentially deposited on the Si substrate 10.

A layered body 400 illustrated in FIG. 14A is made assuming thatsunlight enters from the bottom side as viewed in FIG. 14A.

The Si substrate 10 is the same as the Si substrate 10 of the firstembodiment.

The buffer layer 411 is formed by directly depositing a Ge layer on theSi substrate 10 by a CVD method, and causing a defect in the Ge layer byperforming a thermal cycling process (TCA) so that a structure forrelaxing the difference in the lattice constant of Si and Ge is formedin the Ge layer. Subsequently, a flattening process is performed on thesurface of the Ge layer.

Then, on the Ge layer on which the flattening layer has been performed,a GaAs layer is formed by a MOCVD method. Furthermore, on the GaAslayer, a composition sloped layer made of AlInAs is formed by a MOCVDmethod, so that further lattice relaxation is realized.

Specifically, as the composition sloped layer made of AlInAs, thecomposition ratio of In in the layer is gradually increased (in a slopedmanner), from AlAs (lattice constant: approximately 5.65 Å) having alattice constant that is substantially equal to that of GaAs, to AlInAshaving a lattice constant that slightly exceeds 5.8 Å. Then, thecomposition ratio of In is slightly reduced just before the topmostlayer of the AlInAs composition sloped layer, so that the latticeconstant of the AlInAs composition sloped layer is reduced to 5.8 Å.Then, as the topmost layer of the composition sloped layer made ofAlInAs, a layer having a fixed composition ratio of In is formed, bywhich a lattice constant of 5.8 Å can be obtained.

That is to say, as the buffer layer 411, a layer including a Ge layer, aGaAs layer, and a composition sloped layer made of AlInAs, is formed onthe Si substrate 10.

Furthermore, the contact layer 412 is mainly a layer to be deposited onthe buffer layer 411 for ohmic-connecting with a metal layer 17 (seeFIG. 16) to be formed later. The contact layer 412 is formed bydepositing a GaInAs layer on the buffer layer 411 by a MOCVD method.

The AlInAs cell 420 is a photoelectric conversion cell made of acompound semiconductor, including aluminum (Al), indium (In), andarsenic (As) as raw materials. The AlInAs cell 420 includes an n-layer421 and a p-layer 422. For example, the AlInAs cell 420 is formed by aMOCVD method, by sequentially depositing the n-layer 421 and the p-layer422 on the contact layer 412.

As the dopant of the n-layer 421, for example, silicon (Si) or selenium(Se) may be used. Furthermore, as the dopant of the p-layer 422, forexample, zinc (Zn) or magnesium (Mg) may be used. In the fourth example,the composition of Al, In, and As is adjusted so that the band gap ofthe AlInAs cell 420 becomes 1.9 eV.

The lattice constant of the AlInAs cell 420 is adjusted to approximately5.8 Å, and the AlInAs cell 420 can crystal-grow on the buffer layer 411.

Note that between the n-layer 421 and the contact layer 412 of theAlInAs cell 420 (incident side to the AlInAs cell 420), a window layerhaving a wider gap than that of the AlInAs cell 420 may be formed.Furthermore, above the p-layer 422 (between the p-layer 422 and thetunnel junction layer 413A), a BSF (Back Surface Field) layer having awider gap than that of the AlInAs cell 420 may be formed.

The tunnel junction layer 413A is provided between the AlInAs cell 420and the GaInPAs cell 430, and includes a p-layer and an n-layer ofAlInAs that have been doped at a higher density than that of the n-layer421 and the p-layer 422 of the AlInAs cell 420 and an n-layer 431 and ap-layer 432 of the GaInPAs cell 430. The tunnel junction layer 413A is ajunction layer, which is provided so that a current flows between thep-layer 422 of the AlInAs cell 420 and the n-layer 431 of the GaInPAscell 430 (by tunnel junction).

For example, the tunnel junction layer 413A is formed by a MOCVD method,by sequentially depositing a p type AlInAs layer and an n type AlInAslayer in the stated order on the surface of the AlInAs cell 420. Thep-layer and the n-layer of the tunnel junction layer 413A are preferablyformed with a material having a wider band gap than that of the AlInAscell 420. This is to prevent the light that has been transmitted throughthe AlInAs cell 420 from being absorbed at the tunnel junction layer413A.

Note that the composition of the tunnel junction layer 413A is adjustedso that its lattice constant becomes approximately 5.8 Å, and the tunneljunction layer 413A can crystal-grow on the AlInAs cell 420.

The GaInPAs cell 430 is a photoelectric conversion cell made of acompound semiconductor, including gallium (Ga), indium (In), phosphorus(P), and arsenic (As) as raw materials. The GaInPAs cell 430 includesthe n-layer 431 and the p-layer 432. For example, the GaInPAs cell 430is formed by a MOCVD method, by sequentially depositing the n-layer 431and the p-layer 432 on the tunnel junction layer 413A.

As the dopant of the n-layer 431, for example, silicon (Si) or selenium(Se) may be used. Furthermore, as the dopant of the p-layer 432, forexample, zinc (Zn) or magnesium (Mg) may be used. In the fourthembodiment, the composition of Ga, In, P, and As is adjusted so that theband gap of the GaInPAs cell 430 becomes 1.3 eV.

The composition of the GaInPAs cell 430 is adjusted so that the latticeconstant becomes approximately 5.8 Å, and the GaInPAs cell 430 cancrystal-grow on the tunnel junction layer 413A.

Furthermore, between the n-layer 431 of the GaInPAs cell 430 and thetunnel junction layer 413A (incident side to the GaInPAs cell 430), awindow layer having a wider gap than that of the GaInPAs cell 430 may beformed. Furthermore, above the p-layer 432 (between the p-layer 432 andthe tunnel junction layer 413B), a BSF (Back Surface Field) layer havinga wider gap than that of the GaInPAs cell 430 may be formed.

The tunnel junction layer 413B is provided between the GaInPAs cell 430and the GaInAs cell 440, and includes an n-layer and a p-layer that havebeen doped at a higher density than that of the n-layer 431 and thep-layer 432 of the GaInPAs cell 430 and an n-layer 441 and a p-layer 442of the GaInAs cell 440. The tunnel junction layer 413B is a junctionlayer, which is provided so that a current flows between the p-layer 432of the GaInPAs cell 430 and the n-layer 441 of the GaInAs cell 440 (bytunnel junction).

For example, the tunnel junction layer 413B is formed by a MOCVD method,by depositing a p type AlGaInAs layer and an n type AlGaInAs layer inthe stated order on the surface of the GaInPAs cell 430. The p-layer andthe n-layer of the tunnel junction layer 413B are preferably formed witha material having a wider band gap than that of the GaInPAs cell 430.This is to prevent the light that has been transmitted through theGaInPAs cell 430 from being absorbed at the tunnel junction layer 413B.

Note that the composition of the tunnel junction layer 413B is adjustedso that its lattice constant becomes approximately 5.8 Å, and the tunneljunction layer 413B can crystal-grow on the GaInPAs cell 430.

The GaInAs cell 440 is a photoelectric conversion cell made of acompound semiconductor, including gallium (Ga), indium (In), and arsenic(As) as raw materials. The GaInAs cell 440 includes the n-layer 441 andthe p-layer 442. For example, the GaInAs cell 440 is formed by a MOCVDmethod, by sequentially depositing the n-layer 441 and the p-layer 442on the tunnel junction layer 413B.

As the dopant of the n-layer 441, for example, silicon (Si) or selenium(Se) may be used. Furthermore, as the dopant of the p-layer 442, forexample, zinc (Zn) or magnesium (Mg) may be used. In the fourth example,the composition of Ga, In, and As is adjusted so that the band gap ofthe GaInAs cell 440 becomes 0.9 eV.

The composition of the GaInAs cell 440 is adjusted so that the latticeconstant becomes approximately 5.8 Å, and the GaInAs cell 440 cancrystal-grow on the tunnel junction layer 413B.

Furthermore, between the n-layer 441 of the GaInAs cell 440 and thetunnel junction layer 413B (incident side to the GaInAs cell 440), awindow layer having a wider gap than that of the GaInAs cell 440 may beformed. Furthermore, above the p-layer 442 (between the p-layer 442 andthe contact layer 414), a BSF (Back Surface Field) layer having a widergap than that of the GaInAs cell 440 may be formed.

The contact layer 414 is mainly a layer to be deposited on the GaInAscell 440 for ohmic-connecting with an electrode (metal layer 415A) to beformed later. For example, a gallium indium arsenide (GaInAs) layer isused as the contact layer 414. For example, the GaInAs layer acting asthe contact layer 414 may be formed on the GaInAs cell 440 by a MOCVDmethod.

Note that the composition of the contact layer 414 is adjusted so thatits lattice constant becomes approximately 5.8 Å, and the contact layer414 can crystal-grow on the tunnel junction layer 413B.

As described above, the layered body 400 illustrated in FIG. 14A isformed by sequentially depositing, on the Si substrate 10 and the bufferlayer 411, the contact layer 412 that is the light incident side, theAlInAs cell 420, the GaInPAs cell 430, the GaInAs cell 440, and thecontact layer 414, in the stated order.

Accordingly, the AlInAs cell 420 (1.9 eV) having the widest band gap isprovided closer to the light incident side, than the GaInPAs cell 430(1.3 eV) and the GaInAs cell 440 (0.9 eV).

Furthermore, the GaInPAs cell 430 (1.3 eV) is provided closer to thelight incident side, than the GaInAs cell 440 (0.9 eV).

Next, as illustrated in FIG. 14B, a support substrate 80 is prepared,and a metal layer 415A is formed on the contact layer 414, and a metallayer 15B is formed on the support substrate 80. The metal layers 415A,15B are thin films made of a metal such as gold (Au) or silver (Ag), andmay be formed by a vapor deposition method or a sputtering method.Furthermore, the support substrate 80 may be, for example, a film madeof plastic.

Note that the object formed by forming the metal layer 415A on thecontact layer 414 of the layered body 400 illustrated in FIG. 8A isreferred to as a layered body 400A.

Next, as illustrated in FIG. 15A, the layered body 400A is turned upsidedown, and a bonding layer 16 is used to join the metal layer 415A of thelayered body 400A with the metal layer 15B formed on the surface of thesupport substrate 80.

The bonding layer 16 is formed by, for example, applying, on the surfaceof the metal layer 415A or the metal layer 15B, a joining material suchas a conductive epoxy agent formed, for example, by including silver(Ag) nanoparticles in epoxy resin. The bonding layer 16 is formed by,for example, applying the joining material on the metal layer 415A orthe metal layer 15B by screen printing.

A layered body 400B as illustrated in FIG. 15A is formed by joiningtogether the metal layer 415A and the metal layer 15B by using thebonding layer 16 as described above. In this case, the bonding layer 16is heated so that the metal layer 415A and the metal layer 15B are fusedtogether by the bonding layer 16. The layered body 400B includes thelayered body 400A, the bonding layer 16, the metal layer 15B, and thesupport substrate 80.

Note that the metal layer 415A, the bonding layer 16, and the metallayer 15B are used as the bottom electrode (backside electrode) of thephotovoltaic cell.

Next, the Si substrate 10 and the buffer layer 411 are removed from thelayered body 400B by etching the Si substrate 10 and the buffer layer411 of the layered body 400B illustrated in FIG. 15A. Accordingly, alayered body 400C as illustrated in FIG. 15B is formed. The layered body400C is formed by removing the Si substrate 10 and the buffer layer 411from the layered body 400B illustrated in FIG. 15A.

The etching of the Si substrate 10 and the buffer layer 411 is performedby, for example, etching the Ge layers in the Si substrate 10 and thebuffer layer 411, with a mixed solution (HF:HNO₃:CH₃COOH mixed solution)including hydrofluoric acid (HF), nitric acid (HNO₃), and acetic acid(CH₃COOH). Furthermore, the GaAs layer and the AlInAs composition slopedlayer in the buffer layer 411 are to be etched with a mixed solution(H₂SO₄:H₂O₂:H₂O mixed solution) including sulfuric acid (H₂SO₄),hydrogen peroxide (H₂O₂), and water (H₂O).

In this case, the mixed solution including sulfuric acid (H₂SO₄),hydrogen peroxide (H₂O₂), and water (H₂O) dissolves the GaInAs in thecontact layer 412, and therefore by providing an etching stop-layer suchas a GaInP-layer between the buffer layer 411 and the contact layer412A, the wet etching process can be stopped.

Note that the etching is to be performed by protecting, with aprotective layer, the portions other than the top surface of the Sisubstrate 10 of the layered body 400B (top surface as viewed in FIG.15A), and then impregnating the layered body 400B in the etchingsolution.

That is to say, the side surfaces of the Si substrate 10, the bufferlayer 411, the contact layer 412, the AlInAs cell 420, the tunneljunction layer 413A, the GaInPAs cell 430, the tunnel junction layer413B, the GaInAs cell 440, the contact layer 414, the metal layer 415A,the bonding layer 16, the metal layer 15B, and the support substrate 80and the surface of the support substrate 80 (bottom surface as viewed inFIG. 15A) are to be protected by a protective layer, and then thelayered body 400B is to be impregnated in the etching solution. Aprotective layer will not be used when there is no need for protectionin the above etching process.

Furthermore, in this case, a description is given of an embodiment inwhich both the Si substrate 10 and the buffer layer 411 are removed byetching. However, by removing the buffer layer 411 with the use of asolution for selectively etching only the buffer layer 411, the Sisubstrate 10 may be removed from the layered body 400B. As such anetching solution, for example, a nitrohydrofluoric acid solution (amixed solution of hydrofluoric acid and nitric acid) may be used. Notethat water may be added to the nitrohydrofluoric acid solution to dilutethe solution.

Lastly, a metal layer 17 is formed on the top surface of the contactlayer 412 of the layered body 400C illustrated in FIG. 15B. The metallayer 17 is formed on part of the top surface of the contact layer 412.This is because the metal layer 17 becomes the top electrode of thephotovoltaic cell.

The metal layer 17 may be formed by, for example, a lift off method. Themetal layer 17 is formed by forming a resist on the top surface of thecontact layer 412 illustrated in FIG. 15B, in areas other than the areawhere the metal layer 17 illustrated in FIG. 16 is to be formed,vapor-depositing metal such as Au or Ag from above the resist, and thenremoving the resist.

Note that a contact layer 412C illustrated in FIG. 16 is formed byforming the metal layer 17 (see FIG. 16) on the top surface of thecontact layer 412 illustrated in FIG. 15B, and then using this metallayer 17 as a mask in removing parts of the contact layer 412 (see FIG.15B) other than the part positioned immediately below the metal layer17.

The contact layer 412C may be made by, for example, using a mixed liquidincluding sulfuric acid (H₂SO₄), hydrogen peroxide (H₂O₂), and water(H₂O), as the wet etching solution. In order to stop the wet etchingsolution formed by the mixed liquid including sulfuric acid (H₂SO₄),hydrogen peroxide (H₂O₂), and water (H₂O), a GaInP-layer may be providedbetween the AlInAs cell 420 and the contact layer 412C.

According to the above, a layered body 400D illustrated in FIG. 16 canbe made. The layered body 400D is a photovoltaic cell including threephotoelectric conversion cells, i.e., the AlInAs cell 420 (1.9 eV), theGaInPAs cell 430 (1.3 eV), and the GaInAs cell 440 (0.9 eV). That is tosay, the layered body 400D is a photovoltaic cell made of a compoundsemiconductor.

In the layered body 400D, the buffer layer 411 is an example of a firstbuffer layer, and the AlInAs cell 420, the GaInPAs cell 430, and theGaInAs cell 440 are examples of a first photoelectric conversion cell.Furthermore, the layered body 400B illustrated in FIG. 15A is an exampleof a first layered body, and the Si substrate 10 is an example of afirst silicon substrate.

According to the photovoltaic cell manufacturing method according to thefourth embodiment described above, the layered body 400B (see FIG. 15A)is made by forming the buffer layer 111 on the Si substrate 10, andforming the AlInAs cell 420, the tunnel junction layer 413A, the GaInPAscell 430, the tunnel junction layer 413B, the GaInAs cell 440, and thecontact layer 414, on the buffer layer 411.

Then, after removing the Si substrate 10 and the buffer layer 411 fromthe layered body 400B, the metal layer 17 is formed on the contact layer412, and the contact layer 412C is formed from the contact layer 412.

That is to say, by the photovoltaic cell manufacturing method accordingto the fourth embodiment, the buffer layer 411 for relaxing thedifference in the lattice constant between silicon and the compoundsemiconductor layer, is formed on the inexpensive Si substrate 10, sothat the AlInAs cell 420, the tunnel junction layer 413A, the GaInPAscell 430, the tunnel junction layer 413B, the GaInAs cell 440, and thecontact layer 14 can be formed on the Si substrate 10.

Then, in the manufacturing process, the Si substrate 10 and the bufferlayer 411 are removed from the layered body 400B (see FIG. 15A), by wetetching the Si substrate 10 and the buffer layer 411.

Accordingly, a photovoltaic cell (layered body 400D) made of a compoundsemiconductor can be manufactured at a low cost.

A document (Progress in Photovoltaics 10, 2002, pp. 323-329) describesthat in a triple-junction photovoltaic cell, the combination of 1.9eV/1.3 eV/0.9 eV is preferable.

However, the photoelectric conversion cell made of a compoundsemiconductor included in such a triple-junction photovoltaic cell has alattice constant that is different from that of silicon. Therefore, itis difficult to form the photoelectric conversion cell on a siliconsubstrate.

Meanwhile, by the photovoltaic cell manufacturing method according tothe fourth embodiment, on the Si substrate 10, there is provided thebuffer layer 11 for cancelling out the difference in the latticeconstant of Si and the compound semiconductor, and three photoelectricconversion cells (420, 430, 440) are caused to crystal-grow on thebuffer layer 11.

Thus, according to the fourth embodiment, there is no need to form aphotoelectric conversion cell made of a compound semiconductor on asubstrate having the same lattice constant as that of a compoundsemiconductor such as a GaAs substrate and an InP substrate.

In the fourth embodiment, a predetermined lattice constant that matchesthat of the compound semiconductor is realized by the buffer layer 11formed on the Si substrate 10. Then, three photoelectric conversioncells (420, 430, 440) are caused to crystal-grow on the buffer layer 11.Therefore, a photovoltaic cell including a photoelectric conversion cellmade of a compound semiconductor can be manufactured at a low cost,without using an expensive substrate such as a GaAs substrate and an InPsubstrate.

A structure in which the buffer layer 11 having a predetermined latticeconstant that matches that of the compound semiconductor is formed onthe Si substrate 10, may act as a pseudo-substrate of an expensivesubstrate such as a GaAs substrate and an InP substrate.

The energy conversion efficiency of a triple-junction photovoltaic cellhaving a combination of band gaps of 1.9 eV/1.3 eV/0.9 eV according tothe fourth embodiment, is higher than that of a standard triple-junctionphotovoltaic cell having a combination of band gaps of 1.9 eV/1.4eV/0.66 eV.

Thus, according to the fourth embodiment, a low-cost, high-efficiencyphotovoltaic cell made of a compound semiconductor can be produced,without using a compound semiconductor substrate such as a GaAssubstrate and an InP substrate.

Furthermore, the buffer layer 11 which is for realizing latticerelaxation for eliminating an inconsistency in the lattice constant ofthe Si substrate 10 and the photoelectric conversion cells made of acompound semiconductor (420, 430, 440), includes many defects (latticedefects). Therefore, if the buffer layer 11 is included in the finalproduct, a recombination center may be formed, which may deteriorate theefficiency.

However, the buffer layer 11 is removed in a mid-procedure of thephotovoltaic cell manufacturing method according to the fourthembodiment, and is not included in the final product (layered body400D).

Therefore, by the photovoltaic cell manufacturing method according tothe fourth embodiment, a photovoltaic cell (layered body 400D) havinghigh reliability can be manufactured.

In the photovoltaic cell manufactured by the manufacturing methodaccording to the fourth embodiment, the buffer layer 11 for relaxing thedifference in the lattice constant does not remain in the final productof the photovoltaic cell, and therefore it is possible to manufacture aphotovoltaic cell having higher reliability than the photovoltaic celldescribed in Non-patent Document 1.

Furthermore, according to the fourth embodiment, the Si substrate 10 isremoved, and photoelectric conversion cells (420, 430, 440) made of acompound semiconductor are bonded together with the support substrate 80made of a plastic film, and therefore a flexible and light-weightphotovoltaic cell can be manufactured.

Furthermore, an anti-reflection film may be formed at the light incidentsurface (the part not covered by the contact layer 412C or the metallayer 17) on the top surface of the AlInAs cell 420 of the photovoltaiccell (layered body 400D) illustrated in FIG. 16.

According to an embodiment of the present invention, a photovoltaic cellmanufacturing method is provided, by which a photovoltaic cell made of acompound semiconductor can be manufactured at a low cost

The photovoltaic cell manufacturing method is not limited to thespecific embodiments described herein, and variations and modificationsmay be made without departing from the scope of the present invention.

The present application is based on and claims the benefit of priorityof Japanese Priority Patent Application No. 2012-257829, filed on Nov.26, 2012 and Japanese Priority Patent Application No. 2013-187295, filedon Sep. 10, 2013, the entire contents of which are hereby incorporatedherein by reference.

1. A photovoltaic cell manufacturing method comprising: depositing a first buffer layer for performing lattice relaxation on a first silicon substrate; depositing a first photoelectric conversion cell on the first buffer layer, the first photoelectric conversion cell being formed with a compound semiconductor including a pn junction, and the first photoelectric conversion cell having a lattice constant that is higher than that of silicon; connecting a support substrate to the first photoelectric conversion cell to form a first layered body; and removing the first buffer layer and the first silicon substrate from the first layered body.
 2. The photovoltaic cell manufacturing method according to claim 1, wherein the first photoelectric conversion cell is a multi-junction cell in which at least two photoelectric conversion cells are stacked, which are stacked in series with each other in a stack direction.
 3. The photovoltaic cell manufacturing method according to claim 1, wherein the first photoelectric conversion cell is formed with a material having a lattice constant that is between a lattice constant of GaAs and a lattice constant of InP.
 4. The photovoltaic cell manufacturing method according to claim 1, further comprising: depositing a second buffer layer for performing lattice relaxation on a second silicon substrate; depositing a second photoelectric conversion cell on the second buffer layer, the second photoelectric conversion cell being formed with a compound semiconductor including a pn junction, and the second photoelectric conversion cell having a lattice constant that is higher than that of silicon and different from that of the first photoelectric conversion cell; forming a second layered body by joining the first photoelectric conversion cell of the first layered body and the second photoelectric conversion cell, the second layered body including the first silicon substrate, the first buffer layer, the first photoelectric conversion cell, the second photoelectric conversion cell, the second buffer layer, and the second silicon substrate; and removing the second buffer layer and the second silicon substrate, wherein the removing of the first buffer layer and the first silicon substrate from the first photoelectric conversion cell, includes removing the first buffer layer and the first silicon substrate included in the second layered body, the connecting of the support substrate to the first photoelectric conversion cell, includes connecting the support substrate to the first photoelectric conversion cell included in a third layered body formed by removing the first silicon substrate and the first buffer layer from the second layered body, and the removing of the second buffer layer and the second silicon substrate, includes removing the second buffer layer and the second silicon substrate from the third layered body to which the support substrate is connected.
 5. The photovoltaic cell manufacturing method according to claim 4, further comprising: depositing a first joining layer on the first photoelectric conversion cell after forming the first photoelectric conversion cell; and depositing a second joining layer on the second photoelectric conversion cell, wherein the forming of the second layered body includes joining the first photoelectric conversion cell and the second photoelectric conversion cell by joining the first joining layer and the second joining layer.
 6. The photovoltaic cell manufacturing method according to claim 4, further comprising: depositing a first joining layer on the first photoelectric conversion cell after forming the first photoelectric conversion cell; and depositing a second joining layer on the second photoelectric conversion cell, wherein the forming of the second layered body includes joining the first photoelectric conversion cell and the second photoelectric conversion cell by mechanically joining the first joining layer and the second joining layer by metal.
 7. The photovoltaic cell manufacturing method according to claim 4, wherein one of the first photoelectric conversion cell and the second photoelectric conversion cell is a cell formed with a GaAs lattice matching material or a Ge lattice matching material, and another one of the first photoelectric conversion cell and the second photoelectric conversion cell is a cell formed with an InP lattice matching material.
 8. The photovoltaic cell manufacturing method according to claim 7, wherein the second photoelectric conversion cell is a multi junction cell in which at least two photoelectric conversion cells are stacked, which are stacked in series with each other in a stack direction.
 9. The photovoltaic cell manufacturing method according to claim 1, wherein a part of the first buffer layer is a Ge layer or an SiGe layer.
 10. The photovoltaic cell manufacturing method according to claim 1, wherein the first buffer layer is a strained super lattice layer including GaAs.
 11. The photovoltaic cell manufacturing method according to claim 1, wherein the removing of the first buffer layer and the first silicon substrate, or the removing of the second buffer layer and the second silicon substrate, includes selectively etching a sacrifice layer provided between a buffer layer and a photoelectric conversion cell.
 12. The photovoltaic cell manufacturing method according to claim 1, wherein the removing of the first buffer layer and the first silicon substrate, or the removing of the second buffer layer and the second silicon substrate, includes implanting hydrogen ions in an Si substrate and performing peeling at high temperature.
 13. The photovoltaic cell manufacturing method according to claim 1, wherein the support substrate is a film made of plastic.
 14. The photovoltaic cell manufacturing method according to claim 1, wherein the support substrate is a silicon substrate, and the silicon substrate includes a photoelectric conversion cell made of a silicon semiconductor.
 15. The photovoltaic cell manufacturing method according to claim 14, wherein the first photoelectric conversion cell is formed with a material having band gap energy of 1.4 eV through 1.9 eV. 